1. Field of the Invention
The present invention relates to an analog-to-digital converter, and more specifically, to an analog-to-digital converting module with an adjustable saturation point.
2. Description of the Prior Art
Analog-to-digital converters are widely used in industry for converting analog input voltages into digital output voltages. Analog-to-digital converters are utilized in applications input devices such as pointing devices. Pointing devices are commonly used in conjunction with computers to control the movement of graphical cursers or pointers on display screens and to select objects and operate controls displayed on the screen. Recently, small laptop and notebook computers have become very popular. Laptop and notebook computers may be used in conjunction with a docking station so that a standard keyboard, mouse, and CRT display may be used for the user interface. However, laptop and notebook computers are designed to be used while traveling away from the office or home. In such remote locations, the user does not always have available a flat surface upon which to use a mouse. Accordingly, laptop and notebook computers typically have a built-in pointing device, such as a touchpad.
A touchpad comprises a rectangular surface that is mapped to correspond to a display screen. By touching the location on the touchpad, the user causes the computer to move the pointer to the corresponding location on the screen. Since a typical touchpad is substantially smaller than the screen, accurate positioning of the pointer can be difficult. In order to be useable, a touchpad must be large enough to permit the user to position the pointer accurately. The large size of touchpads makes them relatively expensive.
Please refer to FIG. 1. FIG. 1 is a diagram of a touchpad input system 10 according to the prior art. The touchpad input system 10 contains a touchpad 20 capable of sensing position of a point on the surface of the touchpad 20. The touchpad 20 includes resistors 22 and 24 for generating voltage values according to position of the point on the surface of the touchpad 20. A voltage Vp is applied to each of the resistors 22 and 24, with the other end of the resistors 22 and 24 being connected to ground. The position of the point with respect to each of the resistors 22 and 24 generates voltages Vx and Vy, respectively. The values of the voltages Vx and Vy are directly related to a position of the point on the touchpad 20. In FIG. 1, the point contacts the touchpad 20 at point (x1, y1). The touchpad input system 10 includes a switch 30 for alternatively selecting either an x-axis or a y-axis for measuring a position of the point. A controller 60 outputs a control signal SW_CNT for controlling the switch 30 to sequentially read the analog voltages Vx and Vy generated by the resistors 22 and 24, respectively. A buffer 40 is electrically connected to the switch 30 for impedance matching and for reducing a loading effect in the touchpad input system 10. The voltage value read from the touchpad 20 is stored in the buffer 40. An analog-to-digital converter 50 reads the analog voltage value from the buffer 40, and converts the analog voltage into a digital voltage value. The controller 60 then reads the digital voltage from the from the analog-to-digital converter 50, and position of the point on the touchpad 20 is determined.
The touchpad input system 10 shown in FIG. 1 is a common structure used extensively as an input device for electronics. The resolution of the touchpad 20 is limited by the resolution of the analog-to-digital converter 50. For example, suppose that the analog-to-digital converter 50 has an 8-bit resolution. The touchpad 20 can then have a resolution of 256×256. Increasing the resolution of the touchpad 20 is usually accomplished through the use of a high-resolution analog-to-digital converter. Unfortunately, high costs of high-resolution analog-to-digital converters limit the extent to which the resolution of the touchpad 20 can be increased.